The present invention relates to a process for reducing the free dust in expanded perlite.
Perlite is expanded at temperatures of about 1100.degree. C. in an expansion oven. From the expansion oven, the perlite may be conveyed through a vortex separator which separates and discharges the finest particles, below about 0.2 mm, and passes the larger particles to a bagging operation.
In its transmission from the expansion oven to the bagging operation, the perlite must be cooled from its expansion temperature of about 1100.degree. C. to a temperature which is compatible with handling in the bagging operation. Since modern bagging practice employs plastic sacks, the temperature of the perlite must be reduced to 130.degree. C. or less.
Expanded perlite is used, for example, for sound and heat insulation in walls and ceilings and as a resilient support below flooring. In addition, the oleophilic properties of expanded perlite make it useful for the sorption of oil from the surface of water. The utility of expanded perlite in sorbing oil from the surface of water is improved by a treatment which makes it more water resistant. This is normally accomplished by treating the perlite with a water resistant material such as silicone. Unfortunately, the usual treatment for water resistance increases the tendency of the perlite to break up into dust particles. Consequently the problem is compounded in the usual process of treating perlite for water repellency.
There are methods taught in the prior art for causing the dust in perlite to bond to the larger particles. For example, German Pat. No. 1,269,602 teaches the application of an organic hygroscopic compound to the perlite grains. The hygroscopic compound attracts and holds atmospheric moisture. The moistening of the perlite grains reduces the tendency of the material to dust. Organic hygroscopic compounds such as glycols, lactate and triethanolamine soaps and particularly glycerine are suitable for this application.
Hygroscopic bonding suffers from a number of problems. At low humidity or high temperature, insufficient moisture is attracted to the perlite to bond the dust portion. Furthermore the moisture bonding with hygroscopic compounds is effective only on the finest particles and becomes less effective as the particle size increases. This results in intermediate particle sizes remaining free to create a dust nuisance. This problem becomes especially acute under mechanical stress such as is experienced during the flow of the material through pipes or during transportation in sacks or during discharge from the pipes or sacks.
The dust treatment using hygroscopic compounds is incompatible with the silicone treatment for water resistance. Consequently it is not possible by the known methods to have both good water resistance combined with lack of dusting.
Although mechanical removal of the finest grain fractions is theoretically possible, such mechanical removal of the fraction below about 0.2 mm, for example, by sieving, winnowing or by the use of cyclones presents difficulties in accomplishment and gives a finished product which is not satisfactory for some applications. The cost of separation of large quantities of light materials such as perlite dust is very expensive and time consuming. The separated dust fraction does not contribute to product weight or bulk, and consequently represents a serious loss in product output. An even more serious problem, particularly in the use of expanded perlite as an oil sorbent medium, is that the finest grains which would be removed by a mechanical removal process are the part of the product representing a great proportion of the surface area of the material as well as a great volume of internal pores by which large absorption capacity for oily fluids is provided. It is therefore desirable that removal and discarding of the dust portion of expanded perlite may be avoided at least to some extent in order to improve the product capability for sorption of oily fluids as well as maintaining product output.
Complete removal of the dust portion of expanded perlite is also not desirable in the application of filling under floor surfaces. The finer fraction tends to fill the interstices between the coarser grains and provide a more solid mass which is superior in supporting the floor and in thermal insulation.